Mobile communication systems typically incorporate mechanisms to transmit information about the state of a radio channel. For example, channel state information can be sent as part of the random access mechanism by which mobile terminals can access a network. In one example of a random access mechanism which is under consideration for the UMTS LTE (Universal Mobile Telecommunication System Long Term Evolution) a mobile terminal transmits a signature on an asynchronous random access channel (RACH). The mobile terminal selects a particular signature from a larger set (e.g. of 64), which enables a few bits (e.g. 6) of information to be conveyed. One requirement is that the collision probability (i.e. probability that two mobile terminals use the same signature at the same time) should be sufficiently low. A further requirement is that the resulting waveforms have low cross-correlation.
The bit pattern defining the signature can be considered in terms of one or more data fields. Some examples of the potential contents of the data fields are:                Pseudo-random data (in the form of a temporary mobile terminal identity to reduce the risk of collision)        Channel quality indicator (CQI) or pathloss measurement        Reason for RACH accessThe CQI and Reason fields may also help to reduce collisions if they are sufficiently uncorrelated between terminals.        
The example mechanism described above suffers from competing problems of the limited number of useful data bits which can be sent, and the potential collision probability.